Rapid development of the information industry has increased demand for easily obtaining color hard copies from terminals of information devices, such as electronic computers, facsimile machines, and the like.
Attempts to fulfill this demand include an ink jet system and a heat-sensitive transfer system. However, the ink jet system comprising jetting an ink containing a color material from a narrow nozzle involve a serious disadvantage of poor recording reliability due to a tendency toward obstruction of the nozzle by the color material or other additives. The heat-sensitive transfer system compirses imagewise dissolving an ink on an ink sheet by heating and transferring the dissolved ink to paper. Therefore, formation of a multicolor image uneconomically requires a number of ink sheets, for example, 4 ink sheets for a four-color image. Further, care should always be taken not to suffer an ink shortage in the former system, or not to suffer a shortage of the ink sheets in the latter system. In other words, both of the above systems require complicated control by users.
On the other hand, a heat-sensitive color development system is known as a process with respect to which such complicated control is of no necessity and high recording reliability can be ensured and has, therefore, been rapidly spread in the field of black-and-white facsimiles or printers. This system is characterized by a recording material comprising a support having provided thereon a layer having a color formation function. Because of its easiness with which to record, application of the heat-sensitive color development system to multicolor formation has keenly been demanded.
However, in order to establish multicolor formation taking advantage of the heat-sensitive color development system, it is necessary to integrate a plurality of color formation functions corresponding to the desired number of colors on the same support and to make each of the functions work under control. Many attempts have hitherto been made to effect this idea, but none of them realized sufficient color formation control.
For example, recording materials proposed for multicolor formation include the one in which two color formers capable of developing different colors at different temperatures are used as an admixture in the same heat-sensitive color forming layer, as is disclosed in Japanese Patent Publication No. 69/74, and one in which a color forming layer capable of forming a color at a higher temperature (high-temperature sensitive color forming layer) and a color forming layer capable of forming a color at a lower temperature (low-temperature sensitive color forming layer) are separately laminated on a support, as is disclosed in Japanese Patent Publication Nos. 11989/76 and 133991/77 and Japanese Patent Application (OPI) Nos. 88135/79, 133991/80, 133992/80 and 15540/73. A recording material has also been described further including a decoloring agent, in addition to the above-described high- and low-temperature sensitive color forming layers, said decoloring agent exerting its effect over the color forming components in the low-temperature sensitive color forming layer in the areas corresponding to the image areas of the high-temperature sensitive color forming layer during color formation in the high-temperature sensitive layer, as is described in Japanese Patent Publication Nos. 17866/75, 5791/76 and 14318/82 and Japanese Patent Application (OPI) No. 16188/80.
Further, Japanese Patent Application (OPI) No. 55287/83 discloses a two-color recording material, in which an aqueous solid dispersion containing a leuco dye, an acidic substance capable of causing the leuco dye to develop a color upon heating, a diazo compound, and a coupling component capable of causing the diazo compound to develop a color upon heating is coated on a support and dried to form a heat-senstive recording layer that can develop two colors through heat recording followed by decomposition of the diazo compound by irradiation of light followed by heat recording.
Nevertheless, each of these conventional multicolor heat-sensitive recording materials has its respective drawbacks, and fails in attaining fully satisfactory results.
For instance, in the cases where a recording material comprising a support having provided thereon one or two heat-sensitive color forming layer or layers is recorded at a low temperature and a high temperature to form a low-temperature developed image and a high-temperature developed image that are different in color, respectively, the color of the high-temperature developed image is mixed with that of the low-temperature developed image to a degree varying depending on recording conditions, such as temperature, humidity, recording machines, and the like. Accordingly, it is difficult to constantly obtain images of stable color tone. In addition, when high-temperature recording is effected, there are produced low-temperature zones having the same temperature as employed for low-temperature recording at the periphery of the areas to which a high temperature is applied. Generation of such low-temperature zones around the high-temperature recorded image, the so-called shading or smearing phenomenon, casuses loss of image sharpness.
Although the recording material having a decoloration function prevents color mixing, the problem of color smearing still remains unsolved. Moreover, incorporation of the decoloring agent in the recorded layer causes additional disadvantages, such as color disappearance of an image, reduction in sensitivity of unrecorded materials, and the like, when the recording or recorded materials are left to stand in a high temperature and high humidity atmosphere.
In the recording material containing in the same layer a leuco dye, an acidic color developer, a diazo compound and a coupling component in the form of solid dispersions, the dispersions of these components, though indepedent of each other as water-insoluble dispersions, are inevitably brought into contact with each other within the recording layer, to thereby undergo color formation reactions in the contact phase. As a result, the so-called background fog becomes significant and, in particular, working preservability of unrecorded materials under high temperature and high humidity conditions becomes extremely poor. Additionally, the color formation reaction between the diazo compound and the coupling component, that would smoothly proceed in a basic atmosphere, does not smoothly proceed due to the presence of the acidic color developer for the leuco dye in the same layer, thus resulting in images inferior in sensitivity and hue.